Instrument Development for Single-Particle Albedo Measurements

The ASTER (Aerosol Scattering To Extinction Ratio) instrument simultaneously measures scattering and extinction by single aerosol particles from which the albedo for each particle can be determined. ASTER employs a high-Q laser cavity to amplify loses in the cavity caused by individual particles to produce measurable extinction signals. The instrument collects light in three separate channels representing backward, forward, and wide-angle scattering. The ratio of forward to total scattering provides a proxy measurement for particle size that Mie scattering calculations show to be largely independent of particle refractive index for diameters below about 2 micrometers. Laboratory measurements on particles of known sizes and scattering properties have been used to assess the performance of the instrument and as a guide for ongoing modifications for eventual field deployment. Results from the current version of the instrument will be presented and compared to previous ASTER data to demonstrate improved performance. Data taken from ambient air have shown modes of highly absorbing particles that would not have been evident from bulk measurements. The single-particle nature of the measurements will provide additional information to complement existing methods for measuring aerosol albedos in the atmosphere.